The invention relates to an optical disc player and, more particularly, to a focusing search circuit and a focusing servo circuit in an optical disc player.
FIG. 1 is a block diagram showing a focusing servo circuit in a conventional optical disc player.
In the diagram, reference numeral 1 denotes a disc (for instance, optical recording and reproducing medium such as laser disc, compact disc, or the like); 2 a disc player; 3 an objective lens; 4 a laser diode; 20 a translucent mirror; 5 a two-divided photodetector; 6 a focusing search circuit; 7a and 7b current/voltage converting circuits; 21 a differential amplifying circuit; 12 a focusing error voltage; 8 a comparator; 9 a system microcomputer; 22 a reference voltage; 13 a comparator output signal; 10 a focusing servo loop switch; and 24 a drive circuit of the objective lens 3.
The photodetector 5 and the current/voltage converting circuits 7a and 7b form converting means for converting the reflected light from the disc 1 into the voltage signals. The photodetector 5, current/voltage converting circuits 7a and 7b, differential amplifying circuit 21, switch 10, drive circuit 24, and objective lens 3 form a servo loop when the switch 10 is closed by the microcomputer 9.
The operation will now be described. When the compact disc 1 is loaded into the compact disc player 2, the microcomputer 9 first instructs the drive circuit 4 to perform the focusing search operation to vertically move the objective lens 3 in a manner such that a distance between the objective lens 3 and the signal recording surface of the compact disc 1 is set to the optimum distance.
The focusing search operation is executed in the following manner. A laser beam is emitted from the laser diode 4 and passes through the translucent mirror 20 and the objective lens 3 and is irradiated onto the signal recording surface of the compact disc 1. The reflected light from the signal recording surface is reflected by the mirror 20 and is detected by the two-divided photodetector 5, from which current signals are generated. The two current signals are converted into the voltages by the current/voltage converting circuits 7a and 7b, respectively. A difference between the voltages is obtained by the differential amplifying circuit 21, so that the focusing error voltage 12 is derived.
FIG. 2 shows a change in the focusing error voltage 12 when the objective lens 3 is allowed to approach (is elevated up) from the lower position toward the disc 1. In FIG. 2, when the objective lens 3 reaches a center position 16, the lens 3 is located at the optimum position for the signal recording surface of the disc 1. In this instance, the laser beam is focused onto the signal recording surface of the disc 1 through the lens 3. A technique similar to this focusing servo technique is disclosed in, for example, Iwamura, "Introduction to Video Disc and DAD", Corona Publishing Co., Ltd., pages 136 to 138, November, 1982.
The focusing error voltage 12 from the differential amplifying circuit 21 is compared with the reference voltage 22 by the comparator 8. Since the reference voltage 22 is set so that both of the focusing error voltage 12 and the reference voltage 22 coincide at the timing of the position 16, when both of those voltages coincide, the laser beam is focused onto the signal recording surface. The comparator output signal 13 changes or falls as shown at the timing 16 in FIG. 2, thereby informing the in-focus state of the laser beam to the system microcomputer 9. The system microcomputer 9 turns on the focusing servo loop switch 10, completing the focusing search operation. In the servo loop mentioned above, the system microcomputer 9 subsequently executes the focusing servo operation to fixedly keep the distance between the objective lens 3 and the signal recording surface of the compact disc 1 to the optimum distance.